ALTERNARIA LEAF BLIGHT

CAUSAL AGENT:Alternaria cucumerinaDISTRIBUTION:WorldwideSYMPTOMS:Alternaria leaf blight is a common disease on cantaloupe and of less importance oncucumber, watermelon and squash. Symptoms first appear on the upper leaf surfaceas small, circular, tan spots with white centers. These spots enlarge, turn light brownand form a slight depression. Small leaf veins within the spots darken, resulting in anetted appearance. As the spots enlarge on muskmelon and watermelon, concentricrings develop that are visible only on the upper leaf surface, giving the spot atarget-like appearance. These circular spots can eventually affect the entire leaf.Defoliation may occur, resulting in sunburn damage to the fruit and may lead to adecrease in fruit soluble solids. Severely affected plants also are more susceptibleto heat and wind damage. Infected fruit develop circular, brown sunken lesions. Fruitlesions may develop a dark olive to black-colored powdery mat on the fruit surface.Undetected fruit infection at harvest can result in later losses in transit or storage.

CONDITIONS FOR DISEASE DEVELOPMENT:Alternaria cucumerina survives in crop debris or on weeds and other cucurbit hosts. Diseasespread can occur with rain, irrigation, wind, cultivation, equipment and field workers.This disease is favored by warm temperatures and moisture from dew, rain or overheadirrigation. Infection can be initiated with two to eight hours of leaf wetness, but as the hoursof leaf wetness increase infection level increases. The frequency of rain and the length ofdew periods play a greater role in disease development than the volume of rain that falls.CONTROL:Implement a preventative fungicide spray program. Employ other cultural control measuressuch as crop rotation (two years out of cucurbits), avoid overhead irrigation, thoroughlyincorporate crop debris following harvest and implement a hygiene program for personnel and equipment. For some crops (e.g., cucumber), resistant varieties are available.

ANTHRACNOSE

CAUSAL AGENT:Colletotrichum orbiculareDISTRIBUTION:WorldwideSYMPTOMS:This disease is most commonly found on cucumber, melon and watermelon.Symptoms on leaves begin as water-soaked spots which typically become yellowish inappearance on cucumber and melon or dark brown to black on watermelon. Thesespots eventually turn brown and may expand over the leaf surface. Foliar lesions arenot restricted by leaf veins and often have cracked centers. Infected petioles and stemsmay develop shallow, elongated, tan lesions on melon but the lesions are less obviouson cucumber. Stem lesions on melon can girdle the stem and cause plant wilting.Infected fruit develop circular, sunken, blackish lesions where tiny fruiting bodies(acervuli) may develop. Under humid conditions, the fruiting bodies produce conidiawhich give the lesions a pinkish-salmon color, which is very characteristic of thisdisease. When pedicels of young fruit become infected, the fruit may shrivel and abort.

CONDITIONS FOR DISEASE DEVELOPMENT:Colletotrichum orbiculare can be associated with seed and infected crop debris.Spread of this fungus can occur by splashing rain, overhead irrigation, insects, fieldworkers and equipment. Disease development is favored by warm, humid weather.Optimum temperature for disease development is 24°C (75°F). Late infection of thecrop may result in fruit becoming unmarketable during storage, shipment or display.CONTROL:Implement a comprehensive preventative fungicide spray program. Employ othercultural control measures, such as crop rotation (two years out of cucurbits), avoidoverhead irrigation, thoroughly incorporate crop debris following harvest andimplement a hygiene program for personnel and equipment. Use resistant varietieswhen available.

BLACK ROOT OF CUCURBITS

CAUSAL AGENTS:Phomopsis sclerotioidesDISTRIBUTION:Asia, Europe and CanadaSYMPTOMS:Black root of cucurbit is an important soil-borne pathogen that attacks cucumber,although melon and bottle gourd are also susceptible. Young plants are stunted andwilted. Roots are underdeveloped and rotted, exhibiting a blackened appearance dueto the formation of pseudosclerotia. Leaf senescence increases on affected plants,resulting in appreciable yield losses. Foliar symptoms can appear similar to symptomscaused by vascular wilt fungi (e.g., Fusarium, Verticillium).CONDITIONS FOR DISEASE DEVELOPMENT:Infection is favored by temperatures below 20°C (68°F). However, as temperaturesbecome warmer and/or water requirements increase, disease progression alsoincreases. Phomopsis scerotioides survival in soil is believed to be by means ofpseudostromata and pseudosclerotia. The potential for infection becomes greaterin fields where cucurbits have been grown year after year.CONTROL:Crop rotation has not been shown to be an effective control measure for black root ofcucurbits due to the longevity of pseudosclerotia in soil. Soil fumigation and/or steamsterilization can help reduce fungal populations in soil, but grafting onto a squashrootstock and/or moving production out of soil and into an artificial substrate offer the best options for control.

CERCOSPORA LEAF SPOT

CAUSAL AGENT:Cercospora citrullinaDISTRIBUTION:WorldwideSYMPTOMS:Cercospora leaf spot occurs on all cucurbits but is most common on watermelon,cantaloupe, and cucumber. This disease is usually found only on the foliage, but if theenvironment is suitable, symptoms may also occur on petioles and stems. The fungusis not known to infect fruit. On watermelon, leaf spots manifest on young leaves assmall grey or white spots with black margins. Larger leaf spots which are circular toirregularly circular develop on other cucurbits. The centers of these leaf spots are tanto light brown becoming transparent and brittle with time. Lesions with surroundingchlorotic halos may coalesce and turn leaves yellow. Although defoliation from thedisease may reduce fruit size and quality, serious economic losses are rare.CONDITIONS FOR DISEASE DEVELOPMENT:Conidia of Cercospora citrullina become airborne and may be carried great distances onmoist winds. Infection requires free moisture and is favored by temperatures of 26–32°C(80–90°F). Cercospora citrullina survives on crop debris, volunteers and cucurbit weeds.CONTROL:Incorporate cucurbit debris into the soil to hasten its breakdown and /or removepruning debris entirely from the field. Rotate out of cucurbits for two to three yearsand establish a fungicide spray program to help control this disease.

CHARCOAL ROT

﻿CAUSAL AGENT:Macrophomina phaseolinaDISTRIBUTION:WorldwideSYMPTOMS:This soil-borne fungus can attack roots, stems or fruit in contact with the soil. Onseedlings, black, sunken cankers may appear on hypocotyls at the time of emergence.These cankers may develop a concentric ring pattern, stunt affected plants and causewilt. When older plants are attacked, runners and crown leaves may turn yellow anddie. Typically, a water-soaked lesion will occur at the soil level and extend severalcentimeters up the stem. Brown, water-soaked lesions are also symptomatic of fruitinfection. Amber-colored droplets of exudate may form within the affected area.Eventually, the lesion dries up, turns light tan and microsclerotia form.CONDITIONS FOR DISEASE DEVELOPMENT:Macrophomina phaseolina is seed-borne and can be seed-transmitted. Infection anddisease development are favored by high temperatures. High soil salinity, droughtstress and heavy fruit load can predispose plants to infection. Microsclerotia ininfected host tissue and in soil are the primary propagules and survival structures.Microsclerotia reside in the top 0–20 cm of soil and are able to survive from 2–15years, depending on environmental conditions.CONTROL:Manage irrigation to avoid drought stress. If soil salinity is high, leach to reduce saltbuildup. Drip irrigation may result in higher soil salinity compared to furrow irrigationif salinity of the irrigation water is moderate to high. Destroy or deep-plow all plantdebris at the end of the season. A three-year rotation out of cucurbits to a non-hostspecies may be beneficial. However, this strategy is not as effective at controllingMacrophomina phaseolina as it is with other pathogens due to its wide host rangeand the longevity of the microsclerotia.

DAMPING-OFF

CAUSAL AGENTS:Pythium spp., Rhizoctonia solani, Acremonium spp., Fusarium equiseti, andother fungi.DISTRIBUTION:WorldwideSYMPTOMS:Pre-emergence damping-off: Seeds may rot before germinating or seedlingsmay die prior to emergence.Post-emergence damping-off: Young seedlings develop a rot at the crown; later,the tissue becomes soft and constricted and the plants wilt and fall over.Pythium spp.: Seedlings turn dull green and cotyledons droop. Water-soaked lesionsdevelop on the hypocotyls at the soil line and seedlings wilt and collapse. Seedlingsmay also rot in the soil before emergence.Rhizoctonia solani: This fungus can infect seed, preventing germination. Symptomson young seedlings are similar to those caused by Pythium species. On olderseedlings, a depressed tan to reddish-brown dry lesion may be observed on thehypocotyl.Acremonium spp.: Symptoms develop 7–10 days after seedlings emerge. Infectionbegins where the seed coat remains attached to the hypocotyl. This area turns a lightyellow-brown color. Within two to three days a dry red-brown rot develops, which maylead to seedling death. Surviving seedlings remain stunted.Fusarium equiseti: A dry, reddish-brown rot occurs on the hypocotyl. The funguscauses both pre- and post-emergence damping-off.Thielaviopsis basicola: Lesions begin as grey to reddish areas that almostimmediately turn coal black in color. In wet soil, a frosty coating may cover partsof the black lesion.CONDITIONS FOR DISEASE DEVELOPMENT:Damping-off is generally most severe under conditions of high soil moisture and/or compaction, overcrowding, poor ventilation and cool, damp, cloudy weather. Inaddition, Acremonium root rot is favored by deep planting. Fusarium equiseti attacksmelons which have been seeded into cool, moist soil which later crusts aroundor over the hypocotyls. Seedlings are most susceptible to damping-off prior toemergence or within the first week after emergence. In greenhouses, incompletelypasteurized soil is common source of damping-off fungi and overwatering commonlyexacerbates damping-off.CONTROL:Open Field: In addition to the greenhouse measures described above, avoid soilcompaction, prepare high beds to obtain better drainage and avoid long irrigationperiods. Acremonium root rot can also be minimized by shallow planting in dry soilfollowed by irrigation.Protected Culture: Ensure that substrate/soil consists of components which favordrainage and aeration. Use a reputable substrate/soil supplier. Implement sanitationmeasures for supplies and equipment. Manage irrigation practices to avoid longperiods of high soil moisture. Use high quality seed to help reduce damping-off.Fungicidal soil drenches and seed treatments are available that help managedamping-off. The use of a biological control agent (e.g., Trichoderma harzianum) hasbeen shown to be effective in controlling damping-off pathogens in various cucurbits.

DOWNY MILDEW

CAUSAL AGENT:Pseudoperonospora cubensisDISTRIBUTION:WorldwideSYMPTOMS:Symptoms initially appear as small chlorotic lesions on older leaves, later appearingon the younger leaves. The margins of these lesions are generally irregular on mostcucurbit species. However, on cucumber, lesion margins are defined by the leaf veinswhich give an angular appearance to the lesions. When leaf surfaces remain wet forextended periods, water-soaked lesions develop on the undersides of leaves. Theselesions can appear similar to those caused by Pseudomonas syringae pv. lachrymans.In humid environments, sporangia form on the underside of leaves, giving the appearanceof a whitish-gray to purple fine downy growth. Lesions eventually coalesceand become necrotic, but may continue to expand until the entire leaf dies. Severeinfection results in defoliation, stunting of plants and poor fruit development.CONDITIONS FOR DISEASE DEVELOPMENT:Survival of Pseudoperonospora cubensis between growing seasons is dependent onliving cucurbit hosts. Sporangia may be transmitted considerable distances betweenfields by wind. Within fields sporangia are spread by air currents, splashing water,workers and/or equipment. Fog, dew and frequent rain favor disease development,which can be rapid when temperatures are moderate to warm. High temperatures[>35°C (>95°F)] are not favorable for disease development, but disease developmentmay progress if night temperatures are cool [15–20°C (59–68°F)].CONTROL:Provide adequate spacing between plants to reduce canopy density. Grow varietieswith genetic resistance to Pseudoperonospora cubensis. Implement a preventativefungicide spray program. Regional disease forecasting models have been usedsuccessfully to predict onset of symptoms and to time spray applications for effective control of Pseudoperonospora cubensis.